A DSLAM typically has multiple line ports, where each such port includes a transceiver and can be connected to a subscriber line (also known as a loop). Apart from Digital Subscriber Line (DSL) communication, a DSLAM can be used for testing the quality of the lines it is applied to, typically twisted copper pairs. Such testing can be done using Single Ended Line Test (SELT). A test signal can be sent to the line(s) to be tested, and corresponding echo(es) be received. The echoes are then further analyzed to estimate the length of the total line, interfaces between line sections of different diameter, etc. To get proper measurement values, the DSLAM including its analogue front end (AFE) must be calibrated. Performing a conventional full calibration of a DSLAM takes considerable time. At least two test impedances have to be applied to each port of the DSLAM and at least three echo measurements have to be performed for each port. To get a more accurate calibration, three or more test impedances are needed. This procedure is time consuming and demands handling of every single DSLAM port.
Typically, calibration parameters are derived at “a factory”, e.g. in connection with manufacturing of DSLAMs where a single set of calibration parameters are used for a whole batch of DSLAMs, since it is assumed that component variations etc are small. For best performance, full (individual) calibration should be performed on each port of each DSLAM. Such individual calibration parameters could theoretically be obtained during production or in conjunction with deployment (central office, cabinet, etc) where the latter has the advantage that the calibration would better correspond to the usual working environment of the DSLAMs, as actual temperature, humidity, etc. However, full individual port calibration is not feasible in practice since it is time consuming and would either substantially increase costs or reduce deployment rate. Further, when installing equipment for hundreds or thousands of subscribers, an additional time-consuming calibration step quickly adds up to unacceptable levels of service interruption.
Thus, there is a need for a calibration solution that neither demands a lot of manual handling nor is time consuming during production and/or at the installation site.